21 LEDs from 2xAA batteries

[Josh43]

I know this is technically not a flashlight, but the design is close enough that I hope I can get some help from the obviously very knowledgeable people on this forum.


I have a design that I am trying to iron out for an array of 21 parallel 5mm LEDs.
The batteries are 2xAA NiMH 2000mAh eneloops, and I'm using 100ohm resistors on each led (For a current draw of roughly 400mA)

The main part that makes this work is a 5V up converter ( http://bodhilabs.com/vpackbare.html ), but I am looking around for other options, and the flashlight world seems like it would have the perfect solution.

After looking around, and getting excited about the prices on dealextreme, I realise that I'm kind of lost in this whole world: the main rating is mA output of the boards, as in this unit: http://www.dealextreme.com/details.dx/sku.1885

And it seems as though I can't have all the features I want:
-Step up 2xAA to 3.3V or 5V
-maintain power until the battery is dead (ish), meaning no dimming until the end
-The ability to drive 21LEDs without burning out


I really hope I am wrong about this stuff, because a $3-5 unit that is actually designed to power lights from 2x AA is exactly what I'm hoping for.


Also; Any chance anyone knows a source for a very cheap flashlight barrel? (like $1, plastic, holds 2xAA - I wont be using the emitter or reflector)



Thank you in advance to anyone that gives me some real info

 

[Mr Happy]

I forgot to say...welcome to CPF! :welcome:

Those LED drivers on DX have a mA rating rather than a voltage rating because their main purpose is to drive a regulated and fixed current through an LED in a more efficient manner than using a resistor.

However, one of them would probably work for you if you could find one rated at 400 mA. If your circuit is designed for 400 mA and the LED driver is trying to output 400 mA everything will sort itself out.

Another option you could look at, if you can find one for a reasonable price, is one of the USB cell phone charger thingies that runs off 2AA batteries. These contain a circuit that outputs a regulated 5 V at up to about 500 mA. A typical example is the Energizer Energi To Go: http://www.energizer.com/products/energi-to-go/Pages/ipod-cell-phone-charger.aspx

Unfortunately those Energizer ones sell at a hefty price from most outlets. I once got a few at a bargain price from a Circuit City clearance sale (hint, hint: CC are shutting down a whole bunch of stores right now and might possibly have some more).

Bear in mind that your whole circuit with 21 100 ohm resistors passing 20 mA each is going to waste about 800 mW of power. When you only have 2 AA cells to start with that is quite a lot to lose. I'm not sure I can suggest a much better option though.

 

[Josh43]

Thanks for the greeting

I did see, after looking, that the boards on DX are rated as follows: "3W 1400mA", with the closer version being "1W 350mA", unfortunately I'm not great at doing voltage calcs (I used the led wizard, but I'm learning)..

I did look at those USB chargers, but they are all fairly big (I need this to fit in a tube), unless I build my own minty boost, but that would probly cost me more than $10 per unit..

If anyone does have some input on what I should be doing different with resistors, I'm very interested: I need max brightness and longest battery life - there is no worry on heat and such. I was told I should use resistors even on a 3.3v power source because the LEDs would otherwise soak up as much power as they could.

 

[Marduke]

Well, you could run all 21 LED's parallel off a single 350mA 3.7v boost circuit, with no resistors. It's a more efficient setup. Each LED would end up drawing approximately 16.7mA, which is well within spec for them (assuming we're talking white 5mm's).


BTW, if the goal is just to produce light, you could use a modern single LED rather than 21 5mm's. You'll get much more brightness for the same power, and the whole setup will be simpler.

 

[Josh43]

Well, you could run all 21 LED's parallel off a single 350mA 3.7v boost circuit, with no resistors. It's a more efficient setup. Each LED would end up drawing approximately 16.7mA, which is well within spec for them (assuming we're talking white 5mm's).


BTW, if the goal is just to produce light, you could use a modern single LED rather than 21 5mm's. You'll get much more brightness for the same power, and the whole setup will be simpler.

I'm basically making a light tube (lit hula hoop), as opposed to a flashlight, which is why the array

The LEDs we are talking about all have the same specs:
Reverse Voltage:
5.0V

DC Forward Voltage:
Typical: 3.2V
Max: 3.6V

DC Forward Current:
20mA

I assumed 3.7V would be too much for them, since they spec max at 3.6... Would I have to worry about shortening life?

Also; do you have a source for this type of circuit?

 

[Mr Happy]

Although the Energi To Go thing comes in a plastic shell, it contains a very compact circuit board that can be removed for rewiring if you dismantle it, which might then fit wherever you want it to go.

Another option to consider might be to use 3 or 4 AA cells rather than 2. That way you get the voltage you need more conveniently than using electronics.

You don't need to worry about excess voltage on an LED boost circuit since they are designed to regulate on current, not voltage.

However...I should mention it is not advised to wire 21 LEDs in parallel since due to manufacturing variations some can take more current and some less. They don't self-regulate, so if any LED gets warmer than others it will tend to draw more current, get warmer still and might even go into thermal runaway. To prevent this LEDs should be wired in series, or if in parallel they should have one resistor per LED for current limiting.

 

[MrAl]

Hi there,

I have to agree that if you dont do much with electronics
then you would probably be better off driving with 3 or 4
AA cells rather than just 2 and a boost circuit. The reason is
that boost circuits are not that simple sometimes, and
require the selection of an inductor to get it right.

If you do go with a boost, another idea is to connect the
LEDs in groups of series strings of 3 each, for a total of
7 groups in parallel. This will help balance the current
through each string.

Resistors are a good idea when trying to parallel LEDs because
they also help balance the current through all the LEDs.

 

[Josh43]

The trick is that due to weight, the 2xAA is non-negotiable.
I had also looked at li-ion to drive it directly, but decided against it since this will hopefully be a retail product, and we would rather not have to worry about the .001% change of blow ups.

Also; we are trying to keep costs down, which is why the post: the small board we are currently using works perfectly, it's just that once I saw the sheet of 20 units for $20, I started drooling a little, since it's 1/10th the cost (before shipping).

Also, if I were to do groups of 3, would I then run 1 resistor to each group of 3 leds?

 

[Mr Happy]

Also, if I were to do groups of 3, would I then run 1 resistor to each group of 3 leds?

Yes, that's right. Although when you put more LEDs in series the balancing problem is reduced and it becomes less necessary to have a resistor there. So 7 parallel groups of 3 might be perfectly fine with no resistors if you had a current-regulated supply, or a carefully tuned voltage. To do that you would need a circuit that could supply 10 or 11 V at 140 mA. I've no idea if that could be found off the shelf or if it would be a custom part.

 

[MrAl]

The trick is that due to weight, the 2xAA is non-negotiable.
I had also looked at li-ion to drive it directly, but decided against it since this will hopefully be a retail product, and we would rather not have to worry about the .001% change of blow ups.

Hi again,


Some headlamps use 3xAAA cells. This lowers the weight but yes reduces run
time by quite a bit too, but it's possible rather than 3xAA cells which weigh more.

 

[mikel81]

Lithium 14500? Way lighter and 4v from 1AA size.

 

[Illum]

14500s only have 750mah worth of power to work with...your paying a hefty price for half the tonnage especially if your not drawing alot of current

 

[mikel81]

Good call. I was just thinking weight was his issue, and Lithiums weight next to air.

 

[Illum]

well, I understand where your coming at, I think a 17670 may weigh roughly equivalent to 2AAs...while providing 3.6V [nominal] and has 1600mah in capacity.

Josh43, out of curiosity, why 21 LEDs? [the number gets me]
20ma's a good rating for 5mm LEDs btw :thumbsup:

 

[scott1981]

Another option you could look at, if you can find one for a reasonable price, is one of the USB cell phone charger thingies that runs off 2AA batteries. These contain a circuit that outputs a regulated 5 V at up to about 500 mA. A typical example is the Energizer Energi To Go: http://www.energizer.com/products/energi-to-go/Pages/ipod-cell-phone-charger.aspx

I have purchased it myself and I would like to discourage people from buying it. The reason? Well this device comes with an Energizer disposable lithium battery, however I intended to use it with NIMH. I found that the cut off voltage is higher than 1.3 volts and is you use a rechargeable battery you charged 2 or 3 days ago, it will not start up. I tried with Sanyo 2500`s, Eneloops, etc. Nothing seemed to work. A few days later I put a 2500 cell in the charger and as soon as it was done I put it into the Energi to Go device and it worked.
Conclusion: It will only work with a rechargeable battery right off the charger because of its aparent higher voltage, however it is not a practical tinhg to use, lets say for example when you go camping, you will have your Eneloops already charged, and trust me it won't start up.
I think Energizer is trying to make people buy their Lithiums instead of using NIMHs. There are similar reviews on Amazon about this problem.
It is always a better idea to design your board yourself or buy that board you showed in the first post.

I hope it helps.

Nick.

 

[Josh43]

well, I understand where your coming at, I think a 17670 may weigh roughly equivalent to 2AAs...while providing 3.6V [nominal] and has 1600mah in capacity.

Josh43, out of curiosity, why 21 LEDs? [the number gets me]
20ma's a good rating for 5mm LEDs btw :thumbsup:

I can't remember how we got to 21 - it varies based on the customisation of the size of the product anyway (leds every 5.5", standard diameter worked out to 21 leds). I was glad to come across an ebay vendor that not only has good leds at good prices, but ships free resistors!

I am definitely going to stay away from oem-type lithium cells, due to potential liability issues, but the prototype is really affected by the weight of 2xNiMH, so I'm considering some off the shelf CR2 type batteries and a holder like a flashlight tube, because the appeal of semi-regulated ~3.7v is just getting to be unbearable, and if they are off the shelf, they are the liability of the manufacturer

 

[filibuster]

These may take a little of the soldering fun out of a DYI project, but here's a couple light strands that could work great in a hula hoop with a few modifications.

White
http://www.dealextreme.com/details.dx/sku.6483

RGB
http://www.dealextreme.com/details.dx/sku.6488

 

[uk_caver]

Since each LED will have a series resistor, wouldn't one of the cheap 19-mode voltage-regulated boost converters
http://www.dealextreme.com/details.dx/sku.7882
be a possibility?

It's easy to bypass the mode controller and run the unit as a single-mode driver, and it should be possible to get a combination of LED series resistor value and output voltage which gives a fairly stable 20mA/LED.

It is $3-50/unit in small quantities, but that's still cheaper than the original solution.

 

[dog812]

Hey , i have some DIY instructions on my site to make a LED Hula Hoop using 4 AA's. Check it out for some ideas..
LED Hula Hoop .com